1. Field of the Invention
This invention relates to an automatic data/ voice sending/receiving mode switching device used in a terminal device, by means of which data and voice are bidirectionally sent/received between two parties.
2. Description of the Prior Art
Heretofore data communication between two parties using personal computers, etc. as data communication terminal devices is effected by using MODEM (modulator/demodulator), hybrid circuits and a network control circuit through a telephone network.
On the other hand, in the case where not only data but also voice signals are sent/received, a method by which voice is coded and data thus obtained is transmitted is proposed. However this method can only be applied to a high speed circuit such as digital telephone network. That is, since interruptions occur in the reproduction, of the signals when the signals are transmitted at a speed lower than 8 kb/s, this method can be applied to a high speed circuit such as digital telephone network indicated in an article entitled "INS USER-Network Interfaces" Review of the Electrical Communications Laboratories, Vol. 33, No. 5, 1985, p. 767-774.
Consequently, in practice, it is difficult to apply such a voice transmission method to a low speed circuit such as the conventional analogue telephone circuit. It can be conceived to transmit natural voice in real time in an analogue manner, but in the case where natural voice is transmitted in such a manner, it is necessary to send/receive data and voice while controlling the switching between the two types of signals at each of the terminals.
As a method, by which natural voice is transmitted in real time in an analogue manner, there are known voice/data communication methods published in JP-A 60-254862 and JP-A 60-254891. According to these methods both voice and data communications are effected by using one telephone line. However a problem exists in that it is necessary to use special commands, push-button (PB) signals, etc. for switching between the voice and data signals and thus such methods lack economy and versatility.